1. Field of the Invention
The present invention relates to a machine tool.
2. Discussion of the Background
Vertical lathes include a rotary table that turns about a vertical axis, and perform various processes such as turning, boring, tapping, and milling on a workpiece placed on the rotary table. One example of such a lathe is a multi-tasking lathe including a spindle head pivotable about a horizontal axis (B-axis) disposed on a tool rest movable in a Z-axial direction. The spindle head has a highly rigid structure, and includes a spindle switchable between a rotatable mode and an un-rotatable locked mode. The spindle is switched to the un-rotatable locked mode to mount a turning tool to the spindle. Then, a turning process is performed on the outer portion, inner portion, end surface of the workpiece placed on the rotary table. When the spindle is switched to the rotatable mode, a milling tool, a drilling tool, or a tapping tool is mounted to the spindle to perform milling, drilling, or tapping. However, the spindle head having high rigidity and pivoting about the B-axis has an outer shape having a large dimension. Such a large spindle head produces interference between the workpiece and the spindle, causing various limitations on the process of inner portions of workpieces to be deeply bored. Japanese Unexamined Patent Application Publication No. 2007-000966 discloses a composite machine tool including a tool rest movable in the Z-axial direction and a ram shaft for an internal deep boring process disposed in the tool rest to be movable parallel to a Z-axis, wherein an inner portion of a workpiece placed on a rotary table is subjected to an internal turning process with a turning tool mounted to the ram shaft to be deeply bored.